Conventionally, a metal container with thread, as shown in FIG. 19, is manufactured by drawing, drawing-and-ironing or impact forming from a metallic material such as aluminum. An open mouth part of such a metal container is closed airtight, as a cap is screwed on a threaded part formed in a periphery of the cylindrical mouth part. FIG. 19 is a cross sectional view of a conventional metal container with thread. Denoted at 50 is a container main body, and the container main body 50 is formed by a curled part 51, a mouth part 52 with a threaded part, a tapered shoulder part 53, a trunk part 54 and a bottom part 55, shown in this order from the top. A female screw of a cap 56 and a male screw of the mouth part 52 are fitted with each other, whereby the mouth part 52 of the container main body 50 is sealed up. The symbol 57 denotes packing. In such a container main body 50, as shown in FIG. 20, the shoulder part 53 is formed to have a linear cross sectional shape up to its upper end 53a, and the upper end 53a of the shoulder part 53 is continuous up to a lower end 52a of the mouth part.
Meanwhile, a metal container with thread as shown in FIG. 21 is known and commercially available, where plurality of protruded parts 62 (three in FIG. 21) are formed at schematic equal intervals entirely over a shoulder part 61 of a container main body 60.
However, as shown in FIG. 20, the container main body 50 shown in FIG. 19 has a problem that the strength of the lower end 52a of the mouth part 52 and the upper end 53a (area W) of the shoulder part 53 is weak against pressure forces in a radial direction and an axial direction (pressing forces P, Q) therefore, the area W may be collapsed or buckled during a capping operation to fill content into the container and fit a cap to the container automatically using a machine.
With respect to the container main body 60 shown in FIG. 21, a purpose of forming the plurality of protruded parts 62 entirely over the shoulder part 61 is to mainly achieve a design effect of the metal container with thread and to prevent the shoulder part 61 from getting wrinkled during a necking operation of manufacturing steps. Thus, an improvement in strength at the mouth part and the upper edge of the shoulder part is not intended here, accordingly no actual enhancement of strength is expected.
On the other hand, as shown in FIG. 22, the curled part 51 is formed at the upper end of the mouth part 52 in the conventional metal container with thread to thereby ensure a better strength, safety for a user and sealable effect. In other words, by means of an axial clamping force developed as the female screw of the cap 56 is fitted to the male screw of the mouth part 52, the packing 57 laid on an inner ceiling surface of the cap 56 abuts on a top surface of the curled part 51 formed at the upper end of the mouth part 52, whereby an opening at the mouth part 52 is sealed up. A lower end 56a of the cap 56 is bent along a bead part (annular groove) 52b formed in the mouth part 52 and fitted with the upper area of the bead part to its end.
However, since the packing 57 and the ceiling surface of the curled part 51 are in surface contact, such a closing structure has a problem with a sealing capability if the curled part 51 has a slight dimensional error and the surface contact is accordingly weak. In short, when content develops an internal pressure, the content may leak out between the packing 57 and the ceiling surface of the curled part 51. Particularly, as for a metal container containing, a carbonated beverage such as beer and cola, it is required that the metal container is sealed up without fail again after opened once, do that the quality and the internal pressure of the remaining contained drink are maintained, so that an insufficient sealing capability becomes a problem.
An attempt to improve a sealing capability by strongly tightening the cap 56 and increasing applied pressure between the packing 57 and the curled part 51 to one another invites the area W at the upper end of the shoulder part 53 shown in FIG. 20 to twist more, and therefore, makes it easier for the area W to be buckled. If the tightening force is reduced to prevent such buckling, leakage tends to occur.
On the other hand, in a conventional metal container, an area of the bead part 52b easily collapses when lower end 56a of the cap 56 is plastically deformed along the bead part 52b in the manner as shown in FIG. 22. Further, the cap 56 may be a screw cap with preformed thread or a roll-on type cap put on the mouth part 52 to be pressurized along the male screw of the mouth part to thereby form threads. In the latter case, a side wall of the cap 56 is pressed strongly in the radial direction toward inside, and therefore, the threaded part may easily collapse.
In addition, where the metal container shown in FIG. 19 uses a cap such as a Pilfer Proof cap (PP cap) and a similar Alten cap (also known as a flavor cap or a high-lock cap), to be fractured as opened in order to clearly indicate a user that the container has been unopened, the threaded part, the bead part and the like collapse more easily and the upper end of the shoulder part buckles more easily, since the strength of the cap is high.
The present invention was made in view of the conventional problems described above, and accordingly, a first object of the present invention is to provide a metal container with the threaded part 52 and the lower end 52a of the mouth part as well as the upper end 53a (area W) of the shoulder part 53 having an improved strength in a radial direction and an axial direction. Further, a second object of the present invention is to provide a metal container with thread capable of maintaining a highly airtight condition.